Heat treatment of manganese steel.



UNITED STATES PATENT oEFIoE.

wINEIELn s. POTTER, or NEW YORK, N. Y.

HEAT TREATMENT OF MANGANESE ST EL.

No Drawing.

Specification of Letters Patent.

' To all whom it may concern:

Be it known that I, VVINEIELD S. POTT R, a citizen of the. UnitedStates, residing 1n the borough of Manhattan, city, county, and State ofNew York, have invented certain new and useful Improvements in HeatTreatment of Manganese Steel; and I do hereby declare the following tobe a full, clear, and exact description of the invention, such as willenable others skilled in the art to which it appertains to make and usethe same.

In Letters Patent of the United States granted to me under date ofNovember 8th, 1910, No. 975,371, I have described the production ofshapes for manganese steel which has first been brought to a uniformaust-enite or gamma-iron state by heating to temperatures above 1125 C.and has then been worked to the form contemplated, which may be eitherthe final finished articles, such as a rail or the like, or anintermediate shape such as a bloom, billet, slab, or bar. At thetermination of the working operation, in some of these instances, it mayhappen, because of the exigencies of the mill, or from some other cause,that the worked product may require reheating in order to bring it toits ultimate condition, or its ultimate shape and condition. As aconsequence, the product as it leaves the rolls or. other formingmechanism may, in these instances, be regarded as a preliminary product,which has had the'advantage of the heat treatment and .working describedin my patent referred to, but which will require further treatment inits progress toward the ultimate product or products desired. Forinstance, where the preliminary product is a bloom or billet, and whereit is not feasible to immediately proceed with the rolling of a rail orthe like therefrom, a reheating operation, either from the cold state,or from a temperature intermediate between the heat of rolling and thecold state is essential for the final forming operation. So also, wherethe preliminary product is a slab, with more or less divergence intemperature conditions throughout its mass, which would interfere withits further forming into final products, reheating is likewisefrequently desirable; and, finall when the preliminary product is a baror other commercial shape, or even a rail or the like which may issuefrom the forming rolls at a temperature too low to have terially above950 insure the desired toughness by quenching, a reheating operation isdesirable.

In the treatment contemplated by my United States Letters Patent No.975,371, hereinbefore referred to, the ingot, having first been broughtto the condition, changes in structure, during the rolling to a bloom orother shape, by the breaking down of the austenite grains. The size ofthe resulting broken grains depends upon the percentage of totalreduction effected by the rolls, the amount of reduction in each pass,(that is, thedeepness and suddenness of the reduction), and-also on thefinishing temperature. For example, if in rolling a rail it is finishedwith considerable variations in its temperatures, the grains in thecentral portion of the head of the finished rail will be many timeslarger than the grains in the edge of the base of the rail, for thereason that the pressure is greater and the reduction more severe inproducing the edge of the base than in forming the head, and alsobecause the larger section of'the head, retaining the heat, is finishedat a higher temperature than the edge of. the base. The grains areirregular in form, and

not the uniformity of outline characteristic of'the grains in steelwhich has been heated to 120090. for example, and cooled rapidly withoutmechanical work.

When the bloom or other shape leaves the final pass of the rolls at atemperature ma- C. or thereabout, and if the metal is then permit-ted tocool slowly, changes begin to make themselves manifest in an importantdegree according as the finishing temperature is higher, and increase inmagnitude as the cooling is slower. The first change consists ina'separation of the carbids along the needles or fibers of the austenitestructure within the grain. In manganese steels, containing for example,1.20% carbon this causes .a change in the grain structure from austeniteto martensite. Carbid separations also occur betweenthe grains of themetal, and, if the metal is now cooled either rapidly or slowly toatmospheric temperature, it is weak and brittle. For example if thesteel is slowly cooled from say 1125 C. to 950 (land is then cooledeither slowly or rapidly, it is brittle when cold. Or the metal isbrittle when cold if slowly cooled from above 950 C. to 720 0. Very slowcooling through the range from- Patented Feb. 20, 1912'. Applicationfiled January 9,1912. Serial No. 670,214.

uniform austenite 720 C. to about 640 C. brings about afurtherseparation between the different carbids presentand a still weakercondition in the cold state. Similarly, slow reheating through the rangefrom about 640 C. to 720 0., and slow reheating from say 950 C. to 1125(3., followed by either slow or rapid cooling, produces a weak andeasily cracked or ruptured state of the steel. While in such condition,the manganesesteel should not be rapidly heated or rapidly cooled, as aconsiderable and general cracking, followed by internal oxidation, may,in this way, occur.

I have found that by carefully reheating the steel to between about 975C. and about 1075 C. for a very short time, the separations occurring attemperatures below 950 C. are restored to solution, and-that bycarefully heating to above 1125 C. the Separations occurring above 950C. during slow cooling or slow reheating are also restored to solution;However, the steel reheated to above 1125 C. has a coarse, thoughuniform austenite structure, and, although in a suitable condition forfurther rolling or other working, has not a satisfactory stiffness orstrength even though it were now quenched from the temperature above1125 C. The steel in this coarsely regrained austenite state may begiven a finer grained and strong austenite structure by cooling down andreheating without any longv rest at any temperature between about 640 C.and about 775 C. or between about 950 C. and any higher temperature (forexample, between 950 C. to 1050 C.) either in the cooling or thereheating until brought to a temperature between about 800 C. to 107 5C,

and, if then rapidly cooled. it will be tough and strong.

In the practice of my present invention. I substantially avoid thedifficulties incident to slow cooling and incident to slow reheating ofthe partially formed product. This practice includes the feature that,where it is feasible to do so, the partially formed product should beplaced in the re heating furnace before there has been any substantialseparating out of its carbids at temperatures above 950 C. Also, if, forany reason, the partially formed shape must be cooled after the firstforming operation and from above 950 C. and to a lower temperature (as,for example, if it must be cooled down entirely from 1100 C. until coldbefore it is reheated for further forming) it is then a characteristicfeature of the invention that such cooling is effected rapidly from therolling temperature to below 950 C. and preferably to below 640 (3.,below which temperature carbid separations are not important. Thepractice of my invention is likewise based upon the discovery that thereheating of the rapidly cooled preliminary product should be conductedrapidly through a range beginning with about 400 C. upwardly to themaximum furnace temperature for the subsequent step intended. v

I have found that by the expedient of rapidly cooling the preliminarywrought product, as a preparatory step to the rapid reheating thereof,the separation of carbids and loss of the wrought state of the metal issubstantially avoided, so that the wrought condition in which the brokenaustenite grains are still present and wherein the carbids are uniformlydistributed in the grains, is substantially maintained at thetermination of the rapid cooling operation. So also, I have found thatthis desirable uniform distribution and retention of the carbids withinthe grains of the metal persists during the rapid reheating and thatwhen a temperature of about 800 C. is reached, the broken austenitegrains are reformed, without losing the uniform distribution of thecarbids, into a uniform austenite structure. However, as the temperaturerises, the grain size increases and the strength of the metal when colddecreases, and the temperature for the reheating of finished shapes notfurther to be wrought as above described is preferably limited to atemperature not above 107 5 C. The rapid cooling of thin preliminaryshapes, such as sheet bars, which are afterward to be reheated,may'consist in quenching by immersion in water from the heat of rolling;or the result will be satisfactory if the rapid cooling of the sheet baris effect 100 ed, by means of a light water spray, (for example, untilat a temperature at or below 950 C. but preferably at or below 420 C.)so that the sheet bars will be tough and may be smoothly and exactly cutby shearing.

Heavy wrought shapes such, for example, as thick plate slabs, which areto be reheated for further forming, would be given a condition of severestrain by quenching by immersion in water, and would be liable to 110rupture if immediately reheated rapidly. Therefore, the rapid cooling ofsuch heavy shape is preferably effected as they leave the rolls, byapplying a water shower until the steel is at or about a black heat.

Having thus described my invention, what I claim is: p

1. The method of producing wrought shapes of manganese steel, whichcomprises tenite state; forming; and cooling rapidly to below 950 C.;substantially as described.

2. The method of producing wrought shapes of manganese steel, whichcomprises heating the blank until in a uniform ausitenite state:forming; cooling rapidly to below 950 0.; reheating rapidly; furtherforming; and cooling rapidly to below 950 C. ;-substantially asdescribed.

3. The method of producing wrought heating the blank until in a uniformaus- 1 0 shapes of manganese steel, whichcomprises heating the blankuntil in a uniform austenite state; forming; cooling rapidly to below950 0.; rapidly reheating to between 800 C. and 1075 C. to regram the.steel; and cooling rapidly to below 950 C.; substantially as described.

4. The method of producing wrought shapes of manganese steel, whichcomprises heating the blank until in a uniform austenite state; forming;cooling rapidly to below 950 (1.; rapidly reheating to between 975 C.and 1075 C. for a short time torestore the carbids of the steel tosolution; and cooling rapidly to below 420 0.; substantially asdescribed.

In testimony whereof I afiix my signature;

in presence of two witnesses.

WINFIELD S. POTTER.

' Witnesses: v

JOHN C. PENNIE, MINERVA LOBEL.

